Traditionally, plates for printing presses were produced by hand and were set with moveable type. This method later gave way to automatic machines, such as linotypes. In recent years, printing systems have been introduced which produce printing plates directly from photohardenable material. By exposing such a material to actinic light through a negative image, a selective hardening is obtained which can be used as a printing surface.
Most commonly, the material used for such plates is a photopolymerizable resin. This resin must be spread in an even and uniform layer on a surface on which a negative image has been positioned. Since these resins typically have a high viscosity, it is difficult to provide an even and uniform layer in a short period of time, and the longer the preparation required, the greater the cost of the finished printing plate to the customer.
Most commonly, a layer of photopolymerizable material is applied by depositing a quantity of material on the platemaking surface and then spreading the material by drawing a doctor blade or nip roller along the plate. Various controls have been introduced in order to provide a more uniform deposit of material which is then spread along the platemaking surface. U.S. Pat. No. 3,957,015 shows a polymer flow control system for use in the manufacture of printing plates. The disclosed control system is used in conjunction with a pressurized feed system, and provides multiple spring tensioned dispense controls to assist in the proper distribution of the photopolymerizable material across the width of the platemaking surface. The deposited material is then spread along the lengthwise direction of the platemaking surface. In addition to the multiple dispense controls, footlike extenders at the periphery of the doctor blade are provided to prevent material from spreading beyond the platemaking area of the surface until the material can be spread.
Unfortunately, the use of either a doctor blade or a nip roller, or a combination of these mechanisms, to spread a large amount of viscous material leads to phenomena which prevent the deposition of an accurate and uniform layer. In the case of the use of a doctor blade, a slight cavitation occurs behind the blade as it passes along the plate, and the layer formed is slightly thinner than the height of the doctor blade. In the case of a nip roller, material tends to gather behind the roller as it moves along the plate and a layer formed by such a roller is slightly thicker than the height of the nip roller. The magnitude of either of these effects will depend on such variables as the speed of travel along the platemaking surface of the blade or roller, the viscosity of the material, and the amount of material before the blade or roller which must be moved along.
In addition, it is often customary in this field to apply a sheet of plastic film over the layer of photopolymerizable material to act as a substrate for the finished plate as shown in U.S. Pat. No. 3,937,887 to Akamatsu et al. This sheet is normally laid down by a large roller which follows the doctor blade or nip roller as it travels along the platemaking surface spreading the photopolymerizable material. In the conventional art, variances in the plate thickness as a result of doctor blade or nip roller phenomena during spreading will also result in variances in plate thickness as a result of nip roller phenomena when such a substrate is placed over the layer in this manner.
Mechanisms such as tilt buckets or the bottom opening bucket shown in U.S. Pat. No. 4,056,423, have been used to bring some control to the flow of material onto the platemaking surface, thereby somewhat reducing the amount of material which the doctor blade or nip roller must move along the length of the plate, but there are certain problems associated with these mechanisms. Firstly, such a moveable reservoir must hold the entire amount of material used to make up the layer, usually with some excess to allow for the material's viscosity. Thus, such a reservoir must be of a substantial size, and must be refilled after each plate is made.
In addition, to attempt to keep a relatively constant flow of material during the course of travel along the platemaking surface, the degree of tilt in a tilt bucket, or the width of the opening in a bottom opening bucket, must be varied as the reservoir travels along the surface. This is done to compensate for the speed of travel and the constantly changing relationship between the gravity head of the material in the reservoir and the various other minor forces acting on the material.
To compensate for the irregularities caused by the prior art mechanisms, very often a larger amount of resin than needed was deposited and spread on the platemaking surface. Initially, this resulted in a layer thicker in the center and tapering toward the edges. Even after a wait of 2 or 3 minutes and using an upper weighted flat surface resting on the layer, a layer of uniform thickness might not result. A printing plate made using such a layer would produce dark areas of printing in the center of the plate with lighter areas around the edges.
Recent advances in the art of platemaking from liquid photopolymer resin materials has led to the preparation of multilayer printing plates as disclosed in U.S. Pat. No. 4,266,007. Generally, two layers having different hardnesses are desired, i.e., a soft bottom layer used in combination with a harder top layer can result in improved print quality. For some purposes it may be desirable to have a hard bottom layer and softer top layer. Whichever is desired, it is critical that both layers be of uniform thickness. As indicated above, there are many problems in obtaining a single uniform layer. It should be obvious that those problems are multiplied when one attempts to obtain two uniform layers.